1. Field of the Invention
The present invention relates to a high temperature superconductor (HTS), in particular to a high temperature superconductor known as coated conductor as well as to a process for the production of a coated conductor allowing improved freedom of shaping of the coated conductor.
2. Description of Related Art
Coated conductors, which are also referred to as “second generation super-conductors”, typically have a shape of long length with plane faces such as a tape or stripe. They are composed of a multi-layer structure with a substrate, a superconductor layer and, according to need, one or more buffer layers between the substrate and the superconductor layer with the layers being deposited onto a plane face of the substrate. The buffer layer(s) serve to compensate for the various different properties of the materials used. Typical coated conductor structures require several buffer layers. Finally, onto the superconductor layer a metallic protection layer may be deposited to complete the whole conductor structure.
High temperature superconductors, such as coated conductors, are promising candidates for a plurality of applications for power transmission cables, rotor coils of motors and generators, and windings of transformers, fault current limiters as well as for magnets for medical magnetic resonance imaging (MRI).
It is known in the production of HTS-cables to wind a tape-shaped coated conductor helically around a former.
One major problem in the production of coated conductors is the orientation or alignment of the crystal grains of the superconductor material which should be close to single-crystalline perfection in order to have high current carrying properties such as critical current density (Jc) and critical current (Ic) in the superconducting state. Alignment is also referred to as texture, which means that the orientation of the crystal grains is not random, but has a preferred direction. It is worth distinguishing in-plane and out-of-plane texture, Preferably, the superconductor layer should have a biaxial texture with the crystal grains being aligned both in-plane and out-of-plane.
The quality of biaxial texture is typically expressed in terms of the crystallographic in-plane and out-of-plane grain-to-grain misorientation angle which reflects the degree of inclination of individual crystal grains against each other. The smaller the misorientation angle the better the texture of the layer. The degree of texture can be determined by X-ray diffraction specifying the in-plane and out-of-plane orientation distribution function of the grains of the layer. Based on the X-ray data the values of the full-width-half-maximum (FWHM) of the in-plane phi scan (Δφ) and out-of-plane rocking curve (Δω) can be obtained. The smaller the respective FWHM-value the better the texture. Typically, the misorientation angles should be less than about 10 degrees and, in particular, less than about 5 degrees.
Currently, there are two main approaches to achieve the desired texture. According to the first approach a highly textured buffer layer is deposited onto a polycrystalline, randomly oriented substrate by directed physical coating processes such as ion beam assisted deposition (IBAD). The highly textured buffer layers serve to transfer the desired texture to the superconductor layer grown onto the buffer layer.
According to the second approach a highly textured substrate is used which can be obtained by mechanical working, for example by RABiTS (Rolling assisted biaxial texturing of substrates). Here, the texture of the substrate is transferred to the buffer layer and, then, to the superconductor layer deposited thereon. Examples of metals suitable as substrate are copper, nickel, silver, iron and alloys thereof.
Though not restricted thereto, currently the rare-earth barium cuprate-type superconductors of the formula REBa2Cu3O7-x are conventionally used in the production of coated conductors with x representing the oxygen content in the range appropriate for the particular superconductor material. A preferred member thereof is that one known by the reference YBCO-123 wherein the numerical combination 123 stands for the stoichiometric ratio of the elements Y, Ba and Cu.
Typical buffer layers are ceramic oxides and include lanthanum zirconate, cerium oxide, yttrium-stabilized zirconia (YSZ), strontium titanium oxide, rare-earth aluminates and various rare-earth oxides.
Deposition techniques for the buffer layers as well as of the superconductor layer are well known in the art. Further, tape-like coated conductors and processes for manufacturing thereof are also well known in the art and are widely described.
Due to their ceramic nature the buffer layers and HTS layers are brittle. Further, the quality of the texture is very sensitive to stress. Thus, avoiding damage upon further processing such as shaping of a coated conductor is problematic. This problem of easy damage is increased since the HTS layers typically have thickness as small as about 0.5 to 3 μm only.
For improving the mechanical robustness under bending stress in tape shaped coated conductors Verebelyi et al. in Supercond. Sci. Technol. 16 (2003) 1158 to 1161 and US 2002/144838 A1 of Fritzemeyer et al suggest bringing an over-layer onto the coated conductor, thereby bringing the high temperature superconductor layer in the region of the neutral axis. With this structure bending diameters around the transverse axis down to about 12 mm were obtained without impairment of the critical current. However, considering the width of the tape of about 12 mm the bending diameter on bending around the longitudinal axis would be about 3 to 6 mm which would impose by far more stress onto the material.
DE 197 24 618 A1 of Schippl et al. relates to a corrugated pipe with helical or ring corrugation obtained from a metal tape with a high temperature superconductor layer deposited thereon by forming the tape with the high temperature superconductor layer into a slotted pipe, closing the slot and corrugating the pipe. In this corrugated pipe impairment of the high temperature superconductor layer has been observed in the regions of the wave crests and valleys due to compression and stretching resulting from the corrugated structure. This problem is overcome by the provision of a further metal tape onto the tape with the high temperature superconductor layer with the further tape being significantly thicker than the first tape, for example about the 8 fold. Further, an adhesion promoting layer is provided between the high temperature superconductor layer and the further metal tape which provides sufficient mobility to the structure required for corrugation.
In this structure with the zone of neutral axis being within the further thicker tape a bending radius of about 6 mm (corresponding to a bending diameter of 12 mm) can be obtained without impairment of the high temperature superconductor layer. That is, this structure allows a bending in the order of the bending obtainable in Verebelyi and US 2002/144838 A1 referred to above.
Recently, also coated conductors with circular cross section, also referred to “round coated conductors” have been described wherein the substrate forms a core which is covered by the layer structure. The core may be hollow, such as a tube, or may be solid, such as a rod. For example, such “around coated conductors” and methods for production thereof are disclosed in US 2008/0119365 A1 and EP 1 916 720 A1 which are incorporated herein by reference. A typical method for producing a round coated conductor comprises the steps of forming a plane substrate into a round shape by bending the flat substrate around its longitudinal axis into a slot tube, optionally, texture annealing the shaped substrate, and subsequently depositing thereon the buffer and high temperature superconductor layers.
However, there is the problem that due to the bending stress the texture of the substrate is likely to be impaired. Further, as set out above the risk of damage is increased in case of a coated conductor with already deposited buffer and high temperature superconductor layers.
In the production of round coated conductors the tape-shaped coated conductor has to be bent around its longitudinal axis with very small bending angles corresponding to bending diameters of less than 4 mm down to about 1 mm only. The bending angles in the production of round coated conductors are significantly less than in the production of HTS cables, wherein bending is around the transverse axis as shown in FIG. 1. Thus, the forces generated and the risk of damage by bending the tape-shaped coated conductor around its longitudinal axis are considerably increased compared to the production of cables.